Object 20: Messier 23

Podcast release date: 04 May 2020

Right ascension: 17:57:04.0


Epoch: ICRS

Constellation: Sagittarius

Corresponding Earth location: About halfway between Lima and Rapa Nui in the Pacific Ocean

The open cluster Messier 23, also known as M23 or NGC 6494, is one of the 109 or 110 objects in the Messier Catalogue, which is a very famous catalog of star clusters, nebulae, galaxies, and a few other fuzzy-looking objects. This catalog has been popular with both amateur and professional astronomers. Keep in mind that, when I refer to amateur astronomers, I am not referring to astronomers who are inexperienced or unskilled but instead to people who are not paid for astronomy research but instead who like to stargaze or to use telescopes to observe the sky as a hobby. Some of these amateur astronomers have very good technical skills and very good equipment.

Anyway, back to the Messier Catalogue. The catalog was composed by Charles Messier in the 1700s [1,2]. He was originally interested in spotting comets, which, in an eighteenth-century telescope, look like fuzzy objects that move relative to stars in the background. However, he began spotting fuzzy objects that didn't move relative to background stars, and this really annoyed him, so he began keeping notes on the locations of these objects. While he did go on to discover several comets, most people are much more interested in his catalog of comet rejects, which turned out to be much more impressive. Today, the catalog attracts a lot of attention from amateur astronomers of all skill levels because this list includes some of the brightest types of objects in their class in the sky. In professional research, some of the most intensely-studied astronomical objects are things from the Messier Catalogue, and this includes M1 (the supernova remnant also known as the Crab Nebula), M31 (also known as the Andromeda Galaxy), M42 (also known as the Orion Nebula), and M51 (also known as the Whirlpool Galaxy).

M23 specifically was identified by Charles Messier in 1764 [2]. As I indicated before, M23 is an open cluster, which means that it consists of a number of stars that formed at the same time and that have remained loosely gravitationally bound to each other since forming. Older references indicate that this cluster has somewhere between 100 and 200 stars, but a very recent analysis of open clusters based on data from the GAIA spacecraft identified 789 stars that probably belong to the cluster [3]. The distance to the cluster is about 2360 light years (725 pc) [3]. It's a little tricky to actually describe the size of open clusters because they don't have well-defined edges, but half of the stars fall within a region with a diameter of about the cluster is about 24 light-years (7.4 pc) in diameter. As seen from Earth, this corresponds to 0.6 degrees on the sky [3]. For comparison, the Sun and the Moon as seen from Earth are both about 0.5 degrees in size, so the stars in M23 are spread over a slightly larger area of the sky. In terms of physical units, The cluster as well as all of the stars in the cluster are about 480 million years old [4].

As an amateur astronomy target, M23 attracts a fair amount of attention. It frequently makes its way into lists of the 20 best open clusters to observe [5,6]. However, I have to admit that I don't recall ever observing M23. When I look at this part of the sky with an amateur telescope, I normally get distracted by other things in this part of the constellation Sagittarius like the Lagoon Nebula, which is also called M8, and the Trifid Nebula, which is also called M20. To find M23, I recommend looking for the Teapot, which is a mini-constellation (or technically asterism) in the constellation Sagittarius and then looking about 11.5 degrees north-northwest from Gamma Sagittarii, which is the star at the tip of the spout in the Teapot. Alternately, if you can find the Lagoon Nebula, try looking about 5.5 degrees north-northwest from that. When the average person stretches out his or her hand, that person's thumb is about 2 degrees across, and that person's fist is about 10 degrees across, so you can use that as a way to measure angles on the sky to help you find M23.

The cluster has an apparent magnitude of around 5.5 [7], which is just barely at the threshold of what the average person can see without a telescope at a dark location. The large size and relatively high brightness of the cluster make it easy to see the cluster's brightest stars with just a small telescope (with a diameter of around 80 mm or 3 inches). When viewed this way, you should see a nice concentration of whitish stars. The important thing to do is to observe this cluster with relatively low magnification because the stars are spread out over such a wide area. Zoom in too much, and you may only see a few of the stars in the cluster.

While M23 is a recommended amateur astronomy object, it is not nearly as interesting as a target for scientific analyses. It has appeared in a lot of surveys of open clusters, but it is not often specifically target for detailed study. The last science paper that was written specifically about this cluster was in 1990 [8]. The cluster is thought to lie between two spiral arms in our galaxy, or at least that was the expectation in 1990, and it was thought that the stars in this location may contain fewer elements heavier than hydrogen and helium because the interstellar gas in this region also contains relatively few of these heavy elements. It turned out that the stars have about the same proportion of heavy elements as the Sun, so that hypothesis turned out to be not true. Since then, M23 has mainly been treated as just one of a thousand clusters in various astronomical surveys. Having said that, you shouldn't let the lack of interest from professional astronomers put you off from taking a look at this cluster yourself.


[1] Yeomans, Donald K., Comets: a chronomigical history of observation, science, myth, and folklore, 1991

[2] Pugh, Philip, Observing the Messier Objects with a Small Telescope, 2011

[3] Cantat-Gaudin, T. et al., A Gaia DR2 view of the open cluster population in the Milky Way, 2018, Astronomy & Astrophysics, 618, A93

[4] Bossini, D. et al., Age determination for 269 Gaia DR2 open clusters, 2019, Astronomy & Astrophysics, 623, A108

[5] Bakich, Michael E., The deep sky, 2010, Astronomy

[6] Pasachoff, Jay M., A field guide to the stars and planets, 1992

[7] SIMBAD Astronomical Database, 2020

[8] Sanders, W. L., UBV photometry of NGC 6494 and metallicity condiderations., 1990, Astronomy & Astrophysics Supplements, 84, 615


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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